A homogeneous magnetic field is a basic requirement for NMR-imagings: the higher homogeneity achieved, the better results obtained. A homogeneous magnetic field can be generated by employing various types of basic geometries, such as the Helmholtz couple, an endwise compensated solenoid whose ends are thicker than midpoint, or separate solenoids. For example, Patent publication EP No.-11335 discloses a solution employing four symmetrically positioned solenoids.
Generally, the manufacturing costs of a coil arrangement are considered proportional to the product of power P spent as resistance faults and winding mass m, said product fulfilling the equation: EQU Pm=kd.sup.4 .multidot.B.sup.2,
wherein k is a proportionality factor depending on selected geometry and winding material, d is a dimension describing magnet coil, primarily diameter, and B is a generated magnetic field. It will be noted that the costs depend very much on the size of an apparatus. In terms of costs, therefore, it preferable to make a magnet as small as possible. On the other hand, the homogeneity of a generated magnetic field decreases in the radial direction of solenoid at the same rate the distance from the solenoid axis increases. When imaging of an entire human body is to be effected, the homogeneity area, i.e. an area fulfilling certain criteria set for homogeneity of a magnetic field, has become subject to substantial expansion if compared to minor NMR-analysis equipment for analysing individual samples. In practice, this has been achieved by increasing the size of the magnetic coil arrangement included in the apparatus which in turn, on the basis of the above viewpoints, has resulted in substantial increase of manufacturing costs.
The extent of the homogeneity area of a magnetic field can be effected on by magnetic coil dimensioning factors, which depend on the geometry of a coil arrangement and which are in complicated correlation with each other. In the cited Patent publication EP No.-11335, such dimensioning factors have been utilized to improve the homogeneity of a generated magnetic field. This has produced results wherein the homogeneity of a basic magnetic field remains in category 10.sup.-5 when moving by 13.9% from the diameter dimension of a middlemost coil radially away from the coil centre axis and, accordingly, in category 10.sup.-4 when moving by 17.3% from the diameter dimension of a middlemost coil radially away from the coil centre axis.